Fidelity is one of the most stressed aspects of a relationship. Fidelity or loyalty boils down to the same thing; you both have agreed that your partner has your love, affection and commitment over other potential partners. Fidelity can be a complicated thing though, especially if one partner considers a certain act to be cheating, while the other partner doesn't see the big deal.
Physical fidelity is perhaps the easiest type of fidelity to measure. If two partners are in a committed relationship, it means that they won't have sex with other people. Each relationship is different though, and the two people in it need to establish what is and what isn't considered a breech of physical fidelity. For instance, is it all right for a woman to kiss her ex hello, even if it's a traditional greeting where he comes from? Is it acceptable for a man to accept hugs from other women? These are the sorts of issues that, if left untended, can breed jealousy.
While it's a romantic notion that all you need is love, those who live in the real world know that isn't true. While being with your partner might fulfill some of your emotional needs, there is no relationship where your significant other can be everything to you. Emotional fidelity means that, even though you go and spend time with friends or co-workers, you don't allow your emotional attachment to those people to overcome the emotional fidelity you have to your partner. When emotional infidelity happens, partners have to work out the problems the same way they would if physical infidelity happened.
One of the alternatives to traditional relationships is something called an open relationship. An open relationship functions off of the idea that human beings are not hard-wired to be with only a single partner, so they shouldn't try to be. Two people may admit that they are important to one another and that they love each other, but that's no reason they shouldn't have other close friends or lovers if they so choose, as long as those other lovers don't become more important than their partners. These relationships require communication between all partners.
Jealousy is a reaction to perceived infidelity. Let's say that one partner wants to go out and have lunch with his ex, who his current partner knows he still has feelings for. That sort of situation can lead to jealousy that he's spending time with his ex rather than his current partner. Jealousy can be corrosive to fidelity, and it even may push partners apart because the jealous partner will be seen as controlling or unreasonable.
The best way to maintain fidelity in a relationship is to communicate. Both partners have to learn to trust each other, and they have to be open and honest about their feelings and desires. If partners start keeping secrets or they don't really trust each other, then that is fertile ground for infidelity to occur. If the relationship can't be saved and partners don't want to commit to each other anymore, than it's usually best to end the relationship first, so each person is free to pursue other options.